1. Field of the Invention
The present invention relates to a sacrificial specimen for use in monitoring long-term stress intensity of various structures to predict stress conditions of the structures such as ships, bridges, marine structures and towers which are subjected to natural environments such as winds and waves, railway bridges and loads which are subjected to traffic stress, and cranes used in bays and factories. By means of the sacrificial specimen, a load process of varying stress to structures can be stored in the sacrificial specimen as fatigue damage, and therefore by monitoring the fatigue damage of the sacrificial specimen, it is possible to predict the long-term stress intensity of structures. The present invention also relates to a method of using such a sacrificial specimen for monitoring long-term stress intensity of structures.
2. Related Art Statement
Structures are subjected to various kinds of stresses which vary slowly for a long time. Stresses caused by natural environments such as winds, waves and earthquakes are very complicated. Furthermore, since a large scale structure has a particularly complicated construction, it is sometimes very difficult to grasp stresses on various structural members. Therefore, in order to keep safety of structures as well as to realize a safety designing, it is very important to predict stress conditions of structures by monitoring.
There have been proposed several methods of monitoring stress condition of a structure under use. A stress monitoring method using strain gauges has been widely utilized. In this method, strain gauges have to be secured to portions at which the stress condition should be monitored. Moreover, this method requires an electric power source, switch boxes and strain detecting circuits for strain gauges, and furthermore a recorder is required for recording a history of strain over a rather long time period. Although these components have become rather small, monitoring cost is increased particularly in a large size structure in which a larger number of strain gauges have to be arranged. It should be further noted that wiring operation for connecting various equipments via electrical conductors becomes cumbersome and expensive.
In another stress monitoring method, there are provided precise displacement measuring devices instead of the strain gauges. However, this method still requires an electric power source, a recorder and wiring system.
One of the inventors of the present application has proposed, in U.S. Pat. No. 5,789,680, a sacrificial specimen for use in the fatigue damage monitoring system for predicting a timing at which fatigue damage will actually appear in a structure on the basis of information obtained by monitoring a fatigue damage condition of the sacrificial specimen fixed to the structure. In this known sacrificial specimen, a process of changing stress is not directly monitored, but an accumulation of stress is stored in the sacrificial specimen. This kind of sacrificial specimen is generally provided at a portion of a structure at which stresses are concentrated and a serious fatigue damage might be produced. Therefore, the known sacrificial specimen is constructed to generate a fatigue damage when the sacrificial specimen is subjected to such a large stress by a number of times. It is apparent that such a sacrificial specimen could not be utilized to monitoring a long-term stress intensity of a structure, because the sacrificial specimen is not subjected to a stress having a magnitude which is sufficient for producing a fatigue damage in the sacrificial specimen.